Conventionally, as shown in patent literature 1, a raindrop detection apparatus includes a light emitter that emits lights toward an inner wall of a windshield and a light receiver that measures the lights emitted from the light emitter and reflected by the windshield. The raindrop detection apparatus detects an amount of the raindrops landed on an outer wall of the windshield based on an amount of the lights received by the light receiver.
As described above, the light emitter emits the lights toward an irradiation region on the inner wall of the windshield. However, when the irradiation region on the inner wall of the windshield is increased in order to increase a detection range of the raindrops, a ratio of the raindrops attached to the windshield within the irradiation region to the irradiation region decreases. A detection amount of the raindrops is proportional to an area of the raindrops attached to the windshield within the irradiation region. Thus, a detection accuracy of the raindrops may be degraded. The raindrop detection apparatus disclosed in patent literature 1 includes one light receiver with respect to one light emitter. Thus, one light receiver corresponds to one irradiation region (detection region). Thus, when the irradiation region is broadened, the detection accuracy of the raindrops may be degraded.